The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of common general knowledge in the field.
Input devices based on touch sensing (referred to herein as touch screens irrespective of whether the input area corresponds with a display screen) have long been used in electronic devices such as computers, personal digital assistants (PDAs), handheld games and point of sale kiosks, and are now appearing in other portable consumer electronics devices such as mobile phones. Generally, touch-enabled devices allow a user to interact with the device by touching one or more graphical elements, such as icons or keys of a virtual keyboard, presented on a display, or by writing on a display or pad. Several touch-sensing technologies are known, including resistive, surface capacitive, projected capacitive, surface acoustic wave, optical and infrared, all of which have advantages and disadvantages in areas such as cost, reliability, ease of viewing in bright light, ability to sense different types of touch object, e.g. finger, gloved finger or stylus, and single or multi-touch capability. The most common approach uses a flexible resistive overlay, although the overlay is easily damaged, can cause glare problems, and tends to dim the underlying screen, requiring excess power usage to compensate for such dimming. Resistive devices can also be sensitive to humidity, and the cost of the resistive overlay scales quadratically with perimeter. Another approach is the capacitive touch screen, which also requires an overlay. In this case the overlay is generally more durable, but the glare and dimming problems remain.
In yet another common approach, usually known as ‘infrared’ touch, a matrix of light beams (usually but not necessarily in the infrared) is established in front of a display, with a touch detected by the interruption of one or more of the beams. As shown in FIG. 1 the earliest forms of infrared-style touch screens 2, described for example in U.S. Pat. Nos. 3,478,220 and 3,673,327, included arrays of discrete optical sources 4 (e.g. LEDs) along two adjacent sides of a rectangular input area 6 emitting two sets of parallel beams of light 8 towards opposing arrays of photo-detectors 10 along the other two sides of the input area. If a touch object 12 in the input area blocks a substantial portion of at least one beam in each of the two axes, its location can be readily determined.
In a variant infrared-style touch screen 14 that greatly reduces the optoelectronic component count (and hence component cost), illustrated in FIG. 2 and described in U.S. Pat. No. 5,914,709, the arrays of light sources are replaced by arrays of ‘transmit’ optical waveguides 16 integrated on an L-shaped substrate 18 that distribute light from a single optical source 4 via a 1×N splitter 20 to produce a grid of light beams 8, and the arrays of photo-detectors are replaced by arrays of ‘receive’ optical waveguides 22 integrated on another L-shaped substrate 24 that collect the light beams and conduct them to a detector array 26 (e.g. a line camera or a digital camera chip). Each transmit optical waveguide includes an in-plane lens 28, and likewise each receive optical waveguide includes an in-plane lens 29, to collimate or focus the signal light in the plane of the input area 6, and the touch screen may also include cylindrically curved vertical collimating lenses (VCLs) 30 to collimate the signal light in the out-of-plane direction. As in the touch screen 2 of FIG. 1, a touch object is located from the beams blocked in each axis. For simplicity, FIG. 2 only shows four waveguides per side of the input area 6; in actual touch screens the in-plane lenses will be sufficiently closely spaced such that the smallest likely touch object will block a substantial portion of at least one beam in each axis.
In yet another variant infrared-style touch screen 31 shown in FIG. 3 and disclosed in US 2008/0278460 A1 entitled ‘A transmissive body’ and incorporated herein by reference, the ‘transmit’ waveguides 16 and associated in-plane lenses 28 of the FIG. 2 touch screen are replaced by a transmissive body 32 including a light guide plate 34 and two collimation/redirection elements 36 that include parabolic reflectors 38. Infrared light 40 from a pair of optical sources 4, e.g. LEDs, is launched into the light guide plate, then collimated and re-directed by the collimation/redirection elements to produce two sheets of light 42 that propagate in front of the light guide plate towards the receive waveguides 22. A touch event is detected and its location and dimensions determined from those portions of the light sheets blocked by the touch object. Clearly the light guide plate 34 needs to be transparent to the infrared light 40 emitted by the optical sources 4, and it also needs to be transparent to visible light if there is an underlying display (not shown). Alternatively, a display may be located between the light guide plate and the light sheets, in which case the light guide plate need not be transparent to visible light.
The use of waveguides on the receive side of the touch screens shown in FIGS. 2 and 3 also offers enhanced spatial resolution compared to the ‘discrete component’ touch screen shown in FIG. 1. To explain, only light rays propagating within a narrow range of angles will be focused by each in-plane lens 29 into its associated waveguide, with other light rays rejected.
A common aspect of the infrared-style touch screens shown in FIGS. 1, 2 and 3 is the presence of components around all four sides of the input area 6, which commonly coincides with a display. Whether these components be the arrays of optical sources 4 and photo-detectors 10 in the FIG. 1 device, or the waveguide substrates 18, 24 in the FIG. 2 device, or the waveguide substrate 24 and parabolic reflectors 38 in the FIG. 3 device, their presence limits how narrow the bezels can made. It will be appreciated that the associated ‘bezel width’ on all four sides limits the available display size within a given consumer electronics device, which may be a significant limitation for small devices such as mobile phones. Excessive bezel width can also be a problem for incorporating the additional functionality of infrared touch within an existing device design. Furthermore it will be appreciated that there is a component-related cost associated with having components on all sides of the display as well as extra assembly and manufacturing costs.